A common type of loudspeaker transducer (or driver) has an electromagnetic coil suspended in a strong magnetic field, normally a coil of wire suspended in a gap between the poles of a permanent magnet. When an alternating current electrical audio signal is applied to the voice coil, the coil is forced to move rapidly back and forth due to Faraday's law of induction, which causes a diaphragm or cone attached to the coil to move back and forth, pushing on the air to create sound waves. The electromagnet and the diaphragm vibrate in a direction usually referred to as the driver axis, or the loudspeaker axis. The electromagnet (or voice coil) is housed in a voice coil assembly so that it is free to move reciprocally a pre-determined displacement along the driver axis. Commonly, the voice coil and the diaphragm are circular (in the plane transverse to the driver axis) and there is at least one driver surround (or suspension) which is also circular/annular and disposed generally in the same transverse plane; the driver surround is usually formed of a resiliently flexible material, such as plastic, rubber or felt, and it functions (sometimes together with a spider) to support the electromagnet and the voice coil in position, centering them both on and along the axis, to ensure that the vibrating driver is constrained to move only along the driver axis, and to urge the driver towards a pre-determined point along that axis (the ‘restoring force’). In many cases the surround protrudes along the driver axis in the direction in which the diaphragm propagates sound in a curved “roll”; in other cases the surround protrudes in the opposite direction, in a “reverse roll”. The shape of these rolls is important in determining the audio and mechanical characteristics of the surround; in this application the term ‘roll surface’ is used to define the shape of this surface, in particular it is the shape of a radial cross-section of the surround (i.e. taken in the plane of the driver axis) between the edge of the surround which is fixed to the enclosure and the edge which is fixed to the diaphragm (and/or driver).
As is known, suspension stiffness plays a significant part in determining the resonant frequency of the loudspeaker. The softer the suspension, the lower the resonant frequency, and the more efficiently the loudspeaker can reproduce low frequencies, so the loudspeaker designer chooses a surround material of appropriate stiffness to complement the shape of the surround to optimise performance. The loudspeaker transducer is normally housed in a speaker enclosure or cabinet, with the driver surround also serving to seal the gap between the outer circumference of the voice coil and the enclosure; this is important because it significantly affects the quality of the sound the loudspeaker generates. The materials and shape and size of the enclosure are also important factors affecting the quality of the sound generated.
A vibrating driver diaphragm creates sound in the axial direction away from the loudspeaker, and it also creates sound waves within the enclosure; these internal sound waves have to be catered for also in the design of the loudspeaker to ensure high fidelity, and a common design intended to address this is the well-known port reflex speaker. Another characteristic of such vibrating driver diaphragm loudspeakers is that the movement of the vibrating driver diaphragm out of and into the enclosure changes the volume of the enclosure. As the diaphragm reciprocates it moves into and out of the enclosure, and, where the enclosure is relatively small in relation to the volume swept by the diaphragm (for example an enclosure volume of 4 litres and a diaphragm diameter of 120 mm, giving a volume change of about 2%), this change in volume has significant effects: it gives rise to a change in the back pressure within the enclosure and, where this back pressure acts on the flexible surround it causes the surround to deform. This is shown in the cross-sectional drawings of FIGS. 1A-1C. FIG. 1A shows a surround 1 having a reverse roll 3 which is connected to a diaphragm 5; in this drawing the surround 1 is shown at rest, in FIGS. 1B and 1C the diaphragm 5 has been displaced backwardly (i.e. to the left in the drawing). In FIG. 1B the surround is displaced in free air (i.e. there is no enclosure), whereas in FIG. 1C the surround 1 is fixed to a relatively small (41) enclosure (not shown). The outer edge of the surround 1 (the thickest, uppermost part in the drawings) is fixed (in FIG. 1C it would be fixed to the enclosure). It can be seen that with back pressure in FIG. 1C the outer wall of the surround 1 is pushed significantly inwards such that the edge of the diaphragm 5 collides with it much earlier than is the case in free air (as in FIG. 1C). The deformation of the surround due to the back pressure, and the collision of the diaphragm with the surround adversely affect the sound quality produced by the loudspeaker.
One approach to try and address the deformation caused by back pressure is to increase the thickness of the surround, on the basis that a thicker surround is better able to resist the back pressure, as in WO 1998/007294. However, this increases the mass of the surround, producing a surround having a very nonlinear restoring force, and also gives the driver a very poor frequency response, lowering bass output, breakup frequency and sensitivity. This is illustrated in FIG. 2, which shows the frequency response of two surrounds which are of similar design, but the first surround, with frequency response shown as curve 7, has a thin surround (0.7 mm) and the second surround, with frequency response shown as curve 9, has a thicker surround (1.5 mm). The surrounds producing the frequency curves illustrated have the following characteristics:
Thin surround 7Thick surround 9(0.7 mm)(1.5 mm)Resting stiffness2400N/m14400N/mBreakup frequency1250Hz780HzSensitivity87dB85dBMoving mass18.520.5gBuckling13mm>20mm
There is a further deformation problem which arises with traditional surrounds, which is their tendency to ‘buckle’ when they deform. Such buckling is a result of the geometry of the surrounds (“geometric buckling”) and occurs whether or not the surround is subject to back pressure. In the simple example of a surround having a cylindrical roll surface, in order for the diaphragm to move through a significant axial distance the roll surface must change in shape from a semicircle to a more linear shape; for this to take place, parts of the surround must compress and/or stretch; the surround material is generally not capable of accommodating all the deformation and therefore the surround tends to fold and buckle. Such buckling causes undesirable noise by displacing air and also due to the restoring force changing suddenly when buckling occurs. The pressure deformation of a traditional surround can also lead to geometric buckling occurring much earlier than in free air, as the outer wall of the surround is rapidly forced to a smaller diameter. The buckling causes the restoring force of the surround to change suddenly, increasing distortion. FIG. 3 illustrates the change in restoring force for two similar surrounds, the first shown as curve 11 is of the surround moving in free air (as in FIG. 1B) and the second shown as curve 13 moving when fixed to a relatively small (41) enclosure; it can be clearly seen that the surround has a much more linear restoring force range in the free air example.
There is a need for a surround which can be utilised with a small enclosure but which is resistant to geometric buckling and to uncontrolled deformation caused by back pressure as the diaphragm vibrates, but which is also light.